Bianisotropic Research Articles

Temporal interfaces in complex electromagnetic materials: an overview [Invited

Time-varying metamaterials are currently at the forefront of research, offering immense possibilities for intriguing wave manipulations. Temporal modulations of metamaterials have paved the way for unconventional realizations of magnetless nonreciprocity, wave amplification, frequency conversion, pulse shaping, and much more. Here, we overview the fundamentals and recent advancements of temporal interfaces in isotropic, anisotropic, and bianisotropic materials and metamaterials. Delving into the fundamentals of temporal scattering in media of different material classes, we draw insightful comparisons with phenomena observed at spatial interfaces. We specifically emphasize the potential of time-switched anisotropic and bianisotropic metamaterials in unlocking extraordinary temporal scattering phenomena. Furthermore, an overview of possible platforms to realize time-varying bianisotropic metamaterials is provided. Concluding with a glimpse into the future, we make a research outlook for time-varying anisotropic and bianisotropic metamaterials, highlighting their potential in obtaining exotic photonic time crystals and other dynamic electromagnetic structures.

Reciprocal action of natural bianisotropy and artificial chirality on electromagnetic propagation in medium

Background. The present article investigates the reciprocal action of specific medium effects on electromagnetic waves propagation. The object of study is a moving dielectric, which at rest already demonstrate bianisotropic properties, i. e., it is a synthetic material, e. g., chiral media with Ω-particles. Bianisotropic material equations are the most general for describing the effects of electromagnetic waves interaction with complex medium. Studying and analyzing them is proving to be a notable scientific problem. Natural bianisotropy is a property of simple media under special conditions (state of motion, internal currents and diffusion processes), whereas artificial bianisotropy is an inherent property of the synthetic material itself (composite material, material with different metaparticles).
 Aim. The main goal of the work is to generalize the already available data. On it basis, then, obtain analytical expressions, which can be effectively used for the experiments designing, creating new computational techniques for solving direct and inverse electromagnetic diffraction problems.
 Methods. In this paper, analytical methods are applied to obtain the resulting close-form expressions.
 Results. Three classes of effects have been identified that have a significant reciprocal effect on each other: gyrotropy, spatial dispersion, and temporal dispersion. In this article it was shown that the gyrotropy of the medium has not only a simple additive effect, but under some, specific conditions, can be related to the system emergence.
 Conclusion. The reciprocal action of the spatial dispersion of the moving chiral medium, generally has different scales in range. Temporal dispersion was investigated, which does not have a simple additive property, because even an isotropic medium acquires fundamentally new material properties of bianisotropy when it moves.

Scattering characteristics of a perfect electromagnetic conducting cylindrical object covered with a general Bi-isotropic layer

Abstract An analytic theory has been developed for the scattering of electromagnetic plane wave from a perfect electromagnetic conducting (PEMC) cylindrical object coated with a general bi-isotropic (BI) material. The proposed problem has been solved using cylindrical vector wave function expansion approach along with the application of tangential boundary conditions. Analytic expressions of the scattering coefficients have been derived in their simplest forms. It is seen that by proper selection of admittance of PEMC core, electromagnetic parameters of BI coating, and coating thickness, one can optimize the scattering characteristics for specific applications. It is shown that the specific types of BI and strong chiral-coated PEMC cylinders having certain coating thicknesses can be used to significantly enhance the co-polarized forward scattering while keeping the cross-polarized forward scattering very small. Such types of enhanced co-polarized forward scattering are preferred in point-to-point communication. Some interesting features have been discussed where co-polarized and cross-polarized backscattering may be suppressed, which find applications in radar engineering problems and stealth technology.

Giant enhancement of the axial Casimir force of a rotating particle near hyperbolic metasurface

We show that the axial Casimir force (ACF) acting on a rotating nanoparticle can also appear in the proximity of a hyperbolic metasurface made of graphene ribbons since it was first demonstrated existing near a bi-isotropic material. The ACF can be enhanced by several orders of magnitude compared with that in the bi-isotropic configuration. We numerically demonstrate that the largest ACF exerted on the particle can be achieved when the rotation direction of the nanoparticle is parallel to the hyperbolic metasurface. ACF is also determined by the azimuth angle between the rotating axis and the edge of the metasurface. The dependence of ACF on the chemical potential and ribbon width of graphene is also investigated in detail. ACF increases as the chemical potential increases for fixed ribbon width. For given chemical potential, ACF increases first and then decreases after reaching the maximum value when the ribbon width increases. The results extend the application of hyperbolic metasurfaces in regulating Casimir interactions.

Analysis of optical channel waveguides composed of bianisotropic materials with a strong magnetoelectric effect using a transmission-line formulation

Optical modes in channel waveguides composed of lossy bianisotropic magnetoelectric (ME) materials have been theoretically investigated here. In this study, a semi-analytical method based on the Fourier expansion technique, a well-known transmission-line formulation, is generalized to ME materials in a general case. Using this method, the dispersion diagram, including both phase and attenuation constants, and the field profile of the guided modes in channel waveguides such as rib waveguides are obtained. The presence of ME materials changes the polarization of guided modes in the structure. In all stages of the analysis, the numerical results are calculated and compared with those generated by commercial electromagnetic solvers to verify the computational efficiency and accuracy of the proposed method.

Alternative Finite Element Eigenvalue Formulations for the Simulation of Arbitrarily Bianisotropic Media

We derive two alternative formulations for the implementation of arbitrarily bianisotropic materials for the finite element analysis of eigenmode problems. The e-formulation solves for the Bloch–Floquet periodic envelope of the electric field, and the e–b formulation computes the periodic envelopes of the electric field and magnetic induction. Implementation is carried out in in-house codes and in the Weak Form Module of COMSOL Multiphysics, exhibiting identical results. Extracted dispersion diagrams are compared to the existing solutions in the literature showing perfect agreement. Convergence rates between the proposed formulations are illustrated, with the scaled version of e–b formulation exhibiting faster convergence, due to its linear matrix system and its well-conditioned matrices.

Reflection and transmission of nanoresonators including bi-isotropic and metamaterial layers: opportunities to control and amplify chiral and nonreciprocal effects for nanophotonics applications

Electromagnetic waves reflected from and transmitted through the multilayer nanoresonators including the main layer made of a bi-isotropic material or metamaterial sandwiched between dielectric, epsilon-near-zero or metallic spacer layers have been analytically modeled. The numerical and graphical analysis, based on the exact solution of the electromagnetic boundary problem, confirms opportunities to use such nanoresonators as utracompact polarization converters. The proposed systems are characterized by wide ranges of parameters and significantly reduced (subwavelength) thicknesses. The spacer layers can provide modification, control, and amplification of chiral and nonreciprocal effects for the reflected and transmitted radiation. The concept can be realized for various geometries of dielectric, epsilon-near-zero, metallic, bi-isotropic, metamaterial layers and used to develop new ultrathin, large area, and relatively easy-to-manufacture polarization and other devices for nanophotonics.

Reciprocity condition in synchronously time-periodic bianisotropic materials

In this paper, a sufficient reciprocity condition for general time-periodic modulated bianisotropic media is extracted from first principles. Reciprocity of various cases of significant importance, including stationary bianisotropic media, time-varying (TV) isotropic media, TV anisotropic media, and TV bianisotropic media, are investigated using this condition. We prove that synchronous time modulation of stationary bianisotropic yet reciprocal media (chiral, pseudochiral, and achiral) does not lead to nonreciprocity, unless the modulation function breaks time reversal symmetry. This is in contrast to recently published research. The theoretical results are validated using in-house finite difference time domain simulations.

Electromagnetic waves in layered nanoresonator systems including bi-isotropic and metamaterial layers: analytical model and characteristic features of proper waves

The interaction of electromagnetic waves with multilayers of subwavelength thicknesses (nanoresonators) including the main layer made of a bi-isotropic material or metamaterial sandwiched between pairs of dielectric, epsilon-near-zero or metallic spacer layers has been modeled. The analytical model and numerical analysis are based on the exact and general solution of the electromagnetic boundary problem. The effects of birefringence, dichroism, chirality, nonreciprocity and new possibilities of their control and amplification, the conditions of excitation and features of proper waves inside of the bi-isotropic layer for various parameters of the nanoresonator geometry and components (in particular, metamaterials) and incident radiation are considered. It is shown that bi-isotropic effects in the one-layer chiral and nonreciprocal system can be significantly modified and amplified using the investigated nanoresonators.

Interfacial wave between acoustic media with Willis coupling

Acoustic Willis materials, also known as acoustic bianisotropic materials, exhibit unconventional coupling between pressure and velocity, as well as momentum and strain, which have been shown to enable extraordinary control over the propagation of acoustic waves. Our work first predicts that, under proper conditions, interfacial mode can exist at the interface between two acoustic Willis materials, and the existence of the interfacial wave strongly relies on relative orientations of the Willis coupling vectors of the two materials. To demonstrate our theory, experiments are also conducted utilizing a well-designed adjustable acoustic Willis metamaterial. Our experimental results are in good agreement with simulation and theoretical analysis, although there is unavoidable acoustic attenuation in experiments due to damping effect. This work presents a new functional design with acoustic Willis materials and opens a new route to control acoustic waves.

An Extended k-Surface Framework for Electromagnetic Fields in Artificial Media

The complete understanding of the electromagnetic field characteristics in artificially created bulk or thin media is essential to the efficient harnessing of the multitude of linear and nonlinear effects resulting from it. Due to the fact that recently developed artificial metastructures exhibit controllable electric and magnetic properties that are completely different from natural ones, the spectrum of behavior resulting from subjecting such media to electromagnetic fields has to be revisited. In this paper, we introduce a k-surface framework that offers complete information on the dispersion properties of media with designer electric and magnetic responses with positive and negative values, as well as for the coupling between the two. The extension from the classic k-surface case resides in the consideration of magnetic and bianisotropic materials with positive and negative permittivity and permeability values, as well as the introduction of the chirality coefficient.To illustrate the applicability of our framework, we have investigated the conditions to obtain collinear second harmonic generation in the case of artificial media with positively and negatively valued electric and magnetic responses. As expected, the phase matching tuning curves, defined as the intersections between the k-surfaces at both frequencies, are significantly modified with respect to the classic ones.

Asymmetric lateral beaming of sound in a bianisotropic phononic crystal

Willis materials or bianisotropic acoustic materials that couples pressure to particle velocity provide rich physics for sound manipulation. It has recently been shown that tailored Willis acoustic scatterers with resonance effects can display significant cross-coupling between the monopole and dipole responses. In this talk, we show that the multiple scattering in a bianisotropic phononic crystal composed of Willis acoustic scatterers also leads to interesting coupling behavior. The asymmetric structure in our design is non-resonant in the frequency range of interest. However, the nonlocal effect due to the multiple scattering among the scatterers leads to coupling between quadrupole and dipole responses. The phononic crystal is tuned to match the second-order Bragg scattering with two leaky guided modes. We show that a normally incident wave can be steered to propagate along the positive and negative directions perpendicular to the incident direction with different amounts of energy. Simulation and experimental results will be discussed.

Electromagnetic energy–momentum tensors in general dispersive bianisotropic media

The macroscopic electromagnetic (EM) energy–momentum tensor is one of the most important quantities characterizing the propagation and interaction of light in materials. In recent years, while exotic optical effects in various kinds of bianisotropic materials have been discovered, there still lacks a rigorous analysis of the energy and momentum of EM fields in such general cases. In this paper, using Noether’s theorem and the “Abrahamization” procedure, we obtain generalized Minkowski and Abraham EM energy–momentum tensors, applicable for both arbitrary time-dependent real EM fields and complex-valued analytic signals, in generic lossless bianisotropic media with frequency dispersion. The frequency dispersion of the materials modifies the expressions of EM energy density and Minkowski momentum, making them different from their familiar forms in nondispersive media. Our results reveal that the generalized Minkowski momenta for both real fields and analytic signals are conserved in source-free homogeneous media, while the Abraham momenta, characterizing the centroid motion of light, can change over time, which leads to the counterintuitive phenomenon that wave packets can travel along curved trajectories even in homogeneous bianisotropic media. We also show that the energy–momentum tensor for analytic signals derived from the action principle directly gives the conservation law of time-averaged fields and hence can describe the envelope evolution of waves in quasi-monochromatic approximation.

A well‐conditioned integral equation for electromagnetic scattering from composite inhomogeneous bi‐anisotropic material and closed perfect electric conductor objects

A well-conditioned volume-surface integral equation, called the volume integral equation-combined field integral equation, is applied to analyse electromagnetic (EM) scattering from arbitrarily shaped three-dimensional composite objects comprising both inhomogeneous bi-anisotropic material and closed perfect electric conductors (PECs). The equivalent surface and volume currents are respectively expanded using the commonly used RWG and SWG basis functions, while a matrix equation is derived by the method of moments. Because the magnetic field integral equation is involved in modelling the surface electric current, and the constitutive parameters are all tensors, some new kinds of singularities are encountered and properly handled in the filling process of the impedance matrix. Several numerical results of EM scattering from composite bi-anisotropy and closed PEC objects are shown to illustrate the accuracy and efficiency of the proposed scheme. The validity of the continuity condition of electric flux enforced on the bi-anisotropy-PEC interfaces, which can be used to eliminate the volumetric electric unknowns, is also verified.

New spin-resolved thermal radiation laws for nonreciprocal bianisotropic media

A chiral absorber of light can emit spin-polarized (circularly polarized) thermal radiation based on Kirchhoff’s law which equates spin-resolved emissivity with spin-resolved absorptivity for reciprocal media at thermal equilibrium. No such law is known for nonreciprocal media. In this work, we discover three spin-resolved Kirchhoff’s laws of thermal radiation applicable for both reciprocal and nonreciprocal planar media. In particular, these laws are applicable to multi-layered or composite slabs of generic bianisotropic material classes which include (uniaxial or biaxial) birefringent crystals, (gyrotropic) Weyl semimetals, magnetized semiconductors, plasmas, ferromagnets and ferrites, (magnetoelectric) topological insulators, metamaterials and multiferroic media. We also propose an experiment to verify these laws using a single system of doped indium antimonide (InSb) thin film in an external magnetic field. Furthermore, we reveal a surprising result that the planar slabs of all these material classes can emit partially circularly polarized thermal light without requiring any surface patterning, and identify planar configurations which can experience nontrivial thermal optomechanical forces and torques upon thermal emission into the external environment at lower temperature (nonequilibrium). Our work also provides a new fundamental insight of detailed balance of angular momentum (in addition to energy) of equilibrium thermal radiation, and paves the way for practical functionalities based on thermal radiation using nonreciprocal bianisotropic materials.

Goos-Hänchen shift at the planar interface of NID dielectric and topological insulator

In the literature there exist two models for topological insulator (TI). Model I assumes TI as a nonreciprocal biisotropic material whereas model II accommodates conducting states in the boundary condition. In this paper using an existing mathematical formulation accommodating both models, Fresnel equations and Goos Hänchen shift at the interface of NID dielectric-TI are investigated. Furthermore, impact of NID and/or TI on the GHS of the reflected waves have also been studied and compared with the existing cases.

2-D Circuit-Based Bianisotropic Omega Media

In one dimension, a circuit network representation has been established for bianisotropic materials with an omega response (omega media). However, 2-D circuit-based or transmission-line metamaterials have been restricted to those with only magnetic and electric responses. This article proposes a 2-D circuit-based metamaterial that is equivalent to an omega medium. A lumped element unit cell composed of asymmetric $\pi $ -networks is proposed and validated using a circuit simulator. Circuit analysis of the metamaterial is performed using transmission (ABCD) matrices. A medium equivalency is then established in the homogeneous limit. The application of a 2-D omega medium as an impedance matching layer with phase and power flow control is explored, and a design procedure is developed. The impedance matching layer is then implemented using the proposed lumped element unit cell, and its performance is confirmed using Keysight’s Advanced Design System (ADS).

Tetra-hyperbolic and tri-hyperbolic optical phases in anisotropic metamaterials without magnetoelectric coupling due to hybridization of plasmonic and magnetic Bloch high-k polaritons

In this paper we reveal the physics behind the formation of tri- and tetra-hyperbolic phases in anisotropic metamaterials without magnetoelectric coupling and describe the anti-crossing splitting phenomenon in the hyperbolic dispersion which arises due to the hybridization of the plasmonic and magnetic Bloch high-k polaritons. In hyperbolic metamaterials used for sensing, imaging, and other applications the high-k modes are purely magnetic or electric, which results in purely electric (magnetic) near-fields in response to electric (magnetic) sources. The hybridization predicted in this paper leads to the formation of high-k modes which feature both electric and magnetic fields. This can be used to induce magnetic near fields in response to electric dipole in realistic structures which opens novel avenues in the optical magnetism research. Our work considerably deepens the understanding of the high-k polaritons and the topology of the optical iso-frequency surfaces in k-space and will find applications in optical nano-resolution imaging, emission rate, directivity, and near-field control. To accomplish this, we develop a range of new techniques of theoretical optics for bianisotropic materials, including the quadratic index of refraction operator method, suitable to study the high-k polaritons with finite indices of refraction and the explicit expression for the characteristic matrix in generic bianisotropic media. We introduce the spatial stratification approach for the electric and magnetic responses of anisotropic homogeneous media to analyze the underlying Bloch waves. We believe that the formalisms developed here can be useful for the researchers in the field of theoretical optics of anisotropic and bianisotropic media in the future.

Spin-polarized unidirectional cylindrical waveguide in bianisotropic media.

In this paper, we analyze a cylindrical waveguide consisting of two layers of bianisotropic material with anti-symmetric magnetoelectric coupling tensors. The analysis is carried out in terms of pseudo-electric and pseudo-magnetic fields which satisfy Maxwells' equations with gyrotropic permittivity and permeability tensors. We show that the rotationally symmetric modes of the waveguide are unidirectional with transverse pseudo-electric and transverse pseudo-magnetic modes propagating in opposite directions. These modes are surface waves whose electromagnetic field is concentrated near the interface between the two anisotropic materials. They follow the contour of the interface even in the case of sharp discontinuities and pass through an obstacle without backscattering if the obstacle does not change the polarization of the wave. Higher-order modes of the waveguide are also investigated. Although these modes are hybrid modes and not, strictly speaking, unidirectional, they practically behave as the rotationally symmetric mode.

Classification and characterization of electromagnetic materials

In this paper, we present an efficient method to classify complex electromagnetic materials. This method is based on the directional interaction of incident circularly polarized waves with the materials being tested. The presented method relies on an algorithm that classifies the test materials to one of the following categories: isotropic, chiral, bi-isotropic, symmetric anisotropic or general bianisotropic. The transmitted and reflected fields of right-handed and left-handed circularly polarized waves normally incident from three orthogonal orientations are utilized to determine the reflection/transmission coefficients and complex refractive indices. Both analytical and numerical solutions are used to compute fields of the circularly polarized waves from the arbitrary complex material slab. The complex materials are discriminated accordingly and then classified under an appropriate category. Additionally, new results for material characterization by extracting the scalar/tensorial parameters of bi-isotropic and gyrotropic materials are presented.